(1) Field of the Invention
This invention generally relates to the treatment of particles and, in particular, to a process for the coating or enlargement of particles and an apparatus useful therefor. More particularly, it relates to a granulating process in which priming granules of a particulate material are coated or enlarged by spraying, together with a gas stream, a like or different material in the form of a liquid that is adherent and solidifiable by cooling or drying, and thus causing droplets of the liquid spray to be attached to the surfaces of the granules.
(2) Description of the Prior Art
The necessity of coating or enlarging particles by depositing a material on the surfaces thereof is great in various fields of industry. If the amount of particles to be treated is small, they can readily be coated or enlarged without posing any economic or technical problem.
One process for coating or enlarging particles by depositing a material on the surfaces thereof is disclosed in U.S. Pat. No. 3,231,413. As will hereinafter be described with reference to FIG. 1, this process involves the use of a spouted bed granulator in which priming granules of a particulate material are introduced into a gas stream and caused to encounter, for a very short period of time, with a spray of liquid that is adherent and solidifiable by cooling or drying, and this treatment cycle is repeated until a build-up of desired thickness is obtained on the surfaces of the granules. Specifically, this process causes droplets of the liquid spray to collide with and adhere to the granules being suspended in and conveyed by the gas stream for a very short period of time. However, it will be unsatisfactory if the granules are introduced into the gas stream only once (that is, if it is only once that the individual granules enter the spouted bed where they can encounter with droplets of the liquid spray). The larger the particle diameter becomes, and the greater the amount of liquid to be deposited becomes, the more times the granules must enter the spouted bed.
Referring now to FIG. 1, a spouted bed 22 is formed at the center of a bed 21 of accumulated priming granules (hereinafter referred to simply as a granular bed), extending upward through the granular bed 21. The surrounding annulus of this spouted bed 22 is constituted of the granular bed 21. Preferably, the granules existing at the lower end of the surrounding annulus are smoothly introduced into the spouted bed 22. Thereafter, they are conveyed upward by the gas stream and then allowed to fall on the upper surface of the granular bed 21. Since the entrance of granules into the spouted bed 22 continues to take place at the lower end of the granular bed 21, the granules which have fallen on the upper surface of the granular bed 21 descend gradually through the granular bed 21 and enter the spouted bed 22 again. As described above, the entrance of each granule into the spouted bed 22 must be repeated many times. Moreover, all the granules of the granular bed 21 must be as uniform in number of entrances as possible.
In order that the above-described process of descent through the granular bed 21 and entrance into the spouted bed 22 may be carried on regularly and smoothly, the granular bed 21 is placed in an enclosure 7 having a bottom section of inverted frustoconical or similar shape. The spouted bed 22 is then formed by the action of a gas stream injected from below into the center of the bottom section of the enclosure 7 along its vertical axis. In order to form a stable spouted bed through the granular bed 21, the pressure of the gas stream injected into the bottom portion of the granular bed 21 must be elevated as the depth of the granular bed 21 increases. Accordingly, if the granulating capability of a granulator of the type in which priming granules are enlarged by placing a granular bed in an enclosure having a bottom section of inverted frustoconical shape and forming a spouted bed at the center thereof is enhanced by increasing the depth of the granular bed, the pressure of the gas stream to be injected and hence the amount of energy consumption may be increased to an undue extent. On the other hand, if the granulating capability of a granulator as illustrated in FIG. 1 is enhanced by increasing the diameter of the enclosure 7, the priming granules to be enlarged tend to lack uniformity in the number of entrances into the spouted bed and, therefore, the particle size distribution of the enlarged granules may be widened. This will increase the formation of granules having larger particle diameters than desired and thereby reduce the efficiency. For these reasons, the only possible measure for the mass production of granules on the principle of spouted bed granulation is to use a plurality of granulators.